The ruthenium electroplating baths heretofore proposed have been known for their instability with the undesirable effects of ruthenium oxide precipitation both during storage and actual electrodeposition operations. A serious loss of available ruthenium metal results. Furthermore, the known baths have a tendency to produce ruthenium metal deposits which crack at thicknesses over 0.5 microns. Some baths also require a semi-permeable membrane type cell to prevent the formation of ruthenium tetraoxide at the anode.
It is for these reasons primarily that the ruthenium metal electroplates have not been widely accepted in the industry, although the properties of ruthenium metal as a contact and corrosion resistant material have been known for many years. It would be advantageous therefore to achieve an improved ruthenium electroplating bath; particularly since its current market price is about one-tenth the price of rhodium or gold, each of which could in many instances be replaced by ruthenium metal.
Previous ruthenium metal-containing electroplating baths are described, for example, in U.S. Pat. Nos. 2,057,638; 3,692,641; and 4,189,358. It also has been proposed to improve such baths by utilizing a complex formed by reacting ruthenium metal with sulphamic acid. These complexes have contained from 0.4 to 9 moles of sulphamic acid per mole of ruthenium metal. Unfortunately, the use of these known ruthenium-sulphamic acid complexes has still led to baths which exhibited poor stability in use as well as in storage and, equally deleterious, the readily by-product formation of ruthenium dioxide precipitates. The formation of ruthenium dioxide can also occur by hydrolysis during dilution or during pH adjustment with an alkaline material, e.g. alkali metal hydroxide, ammonium hydroxide, and the like. Representative U.S. patents which diclose complexes of sulphamic acid and ruthenium metal include U.S. Pat. Nos. 3,576,724; 3,625,840; 3,630,856; 3,793,162; 4,082,624; and 4,189,358.